Coal-derived liquid fuels have obvious advantages over other coal forms as energy sources. However, coal liquid fuels are known to deteriorate with time, the molecular weight, oxygen content, asphaltene (gum) content, and viscosity all increasing on processing and storage. See, for example, Frankenfeld, J. W. and Taylor, W. F., Am. Chem. Soc., Div. Fuel Chem. Prepr., 23 (4), 205 (1978); Lin, Y. Y., Anderson, L. L. and Wiser, W. H., Am. Chem. Soc., Div. Fuel Chem. Prepr., 19 (5), 2 (1974); Finseth, D., Hough, W., Queiser, J. A. and Retcofsky, H. L., Am. Chem. Soc., Div. Petrol. Chem. Prepr., 24 (4), 979 (1979); Brown, F. R. and Karn, F. S., Fuel, 59, 431 (1980); Kershaw, J. R. and Gray, D., Fuel, 59, 436 (1980); Brinkman, D. W., Bowden, J. N., Frankenfeld, J. W. and Taylor, W. F., Am. Chem. Soc., Div. Fuel Chem. Prepr., 25 (3), 110 (1980); Hara, T., Jones, L., Li, N. C. and Tewari, K. C., Fuel, 60, 1143 (1981).
Such deterioration adversely affects fuel handling, burner performance, and burning efficiency. Gum formation is a general phenomenon and presents serious practical problems. In the extreme case of solidification this could result in very serious problems and complete loss of the advantages of a liquid fuel.
Recent work by N. C. Li and coworkers strongly suggests that metal-catalyzed reactions of phenols with dioxygen are largely responsible for storage instability and deterioration of coal liquids. See, for example, White, C. M. and Li, N. C., Anal. Chem., 54, 1564 (1982); White, C. M. and Li, N. C., Anal. Chem., 54, 1570 (1982); Jones, L. and Li, N. C., Fuel, 62, 1156 (1983); White, C. M., Jones, L. and Li, N. C., Fuel, 62, 1397 (1983). The investigations of these workers have shown that phenolic constituents in coal liquid fractions are coupled in the presence of dioxygen to form various polymeric ethers, and have provided direct evidence for the presence of dimers, trimers, tetramers, and pentamers derived from phenols shown to be present initially.
In the past, attempts to stabilize coal liquid fractions have focused on cost-ineffective processes to remove phenolic materials, utilizing base extraction or conversion of phenolic materials to corresponding ether derivatives. Derivatization may be carried out in situ or after separation of the phenols from the coal liquid fraction, and in the latter case the ether derivatives have sometimes been returned to the coal liquid. These processes are shown in U.S. Pat. Nos. 4,277,326; 4,277,327; 4,299,691; 4,319,981; 4,353,792; 4,382,855; 4,406,780. It is possible to precipitate gums from aged coal liquid fractions by the addition of nonpolar hydrocarbons, as shown by Jones, L. and Li, N. C., Fuel, 62, 1156 (1983). Another possible approach to stabilization of coal liquid fractions is to upgrade such fractions by further hydrogenation under much more severe conditions than were used in the original liquifaction process, as shown by Young, L. J. S., Li, N. C., and Hardy, D., Fuel, 62, 1397 (1983). The objective then is to remove all potentially oxidizable functional groups from the fuel constituents while not creating new products which are even more highly reduced and still likely to be air sensitive. This approach has also been employed in U.S. Pat. No. 4,319,981.
All of the above-described processes for stabilizing coal liquid fractions constitute additional steps in the processing of such liquids, and require plant facilities, chemicals, and energy. As a result, they inevitably increase the cost of producing stable coal liquid fractions, to a point at which these processes become prohibitively expensive.
To avoid the expense of additional processing steps in the production of stable coal liquid fractions, it is most desirable to have a stabilizing process in which stabilization is achieved by adding one or more appropriate inexpensive stabilizing agents which retard or prevent viscosity increases and gum deposition. Such a process is the subject of this invention.